Why Does Ice Have a Lower Density Than Water?

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Quick Answer

The hydrogen bonding gives water a structure with considerable space between the molecules, making it expand in size and become less dense in a solid state than in a liquid one. Because water is denser than ice, ice cubes float on the surface of water. Hydrogen bonds link each water molecule to an average of 3.4 other water molecules, and the lattice that forms when water freezes has a lot of empty space inside it.

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Water is the only liquid that becomes less dense as a solid. Density refers to the mass of a material per unit of volume, and density does change along with temperature. Even though the mass remains constant, the space or volume that it takes up also changes with temperature. As temperatures go up, so does the vibration of the material's molecules, leading to this drop in density.

If someone sets ice in water or leaves it exposed to air, it starts melting sooner in the water, assuming that the water and air both have identical temperatures. The liquid water has molecules more tightly packed than those in the air, giving a higher level of contact with the ice and a swifter heat transfer rate. Add that to the fact that liquid water has a higher rate of heat transfer, and the explanation becomes clear.